Is it a condition that the vowed calf be two years old when slaughtered, or is it permissible to slaughter it when it is one or two months old?

If the mother's vow was merely to slaughter a calf without intending to seek proximity to God or give charity, then she is not obligated to fulfill it. However, if she intended it as a means of drawing closer to Allah or giving charity to the poor, then she is obligated to fulfill it once its condition is met.

If she vowed a specific calf, it must be slaughtered regardless of its age. But if she did not specify a particular calf, the preponderant opinion is that she must slaughter what is permissible for Udhiyah (sacrificial animal), which is an animal that has reached two years of age or more and is free from defects.